Qt
Folly
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Qt | Folly | |
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26 | 88 | |
2,238 | 26,949 | |
2.5% | 1.0% | |
10.0 | 9.8 | |
6 days ago | about 8 hours ago | |
C++ | C++ | |
GNU General Public License v3.0 or later | Apache License 2.0 |
Stars - the number of stars that a project has on GitHub. Growth - month over month growth in stars.
Activity is a relative number indicating how actively a project is being developed. Recent commits have higher weight than older ones.
For example, an activity of 9.0 indicates that a project is amongst the top 10% of the most actively developed projects that we are tracking.
Qt
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Current Issues With The Qt Project - From The Outside Looking In
Qt mono repo : .. you could check out all submodules and simply use CMake to exactly achieve this. A mono repo also means that if I only use qtbase and declarative, I would need to have all submodules in there? - No
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Why is building a UI in Rust so hard?
For e.g. if you’re writing a framework, you need to interface with Cocoa on MacOS to draw windows, which only provides an Objective C or Swift interface. You can look at the Qt source code and see how they do it: https://github.com/qt/qtbase/tree/067b53864112c084587fa9a507eb4bde3d50a6e1/src/plugins/platforms/cocoa
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Fish (shell) porting to Rust from C++
That's because Qt 6 wholeheartedly converted to CMake for you. (At least it is better than qmake.) In order to support this Qt has this large battery of CMake files [1]. Qt is of course a clear outlier, but you can't expect the same level of support from every other library you want. My points about "anything exotic" still stand.
- A question about how GUI libraries are written.
- Ask HN: Why is there no performant remote desktop for Mac/Linux?
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Post-mortem of a long-standing bug in video Game Path Of Exile, which was caused by a stale pointer
I don't see any connect in https://github.com/qt/qtbase/blob/dev/src/corelib/tools/qsharedpointer_impl.h, and QPointer isn't a QObject (though I don't know if the latter is actually necessary for signal-slots). One (unreliable) way to test is to see if a QPointer fails to be nulled out when the QObject is blocked by a QSignalBlocker. Alternatively I'd set a data breakpoint on a QPointer and try it out. But I don't have the time right now.
Interestingly Qt has QPointer which nulls itself out when the target T is deleted. It's convenient when I want weak references to GUI objects (though you have to be careful to check for its presence after every time you call code which could possibly delete it, I usually call it QPointer maybe_foo). However, from my brief look at the source (link), it's implemented in terms of qsharedpointer.h-> qsharedpointer_impl.h and QtSharedPointer (not sure how it works, but there's probably overhead going on). I wonder how it works, and compares to generational indexes or Vale's generational references (link).
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Why is 5.15.2 the last version in git?
The tag is right there: https://github.com/qt/qtbase/tree/v5.15.4-lts-lgpl
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[Weekly] What is everybody working on? Share your progress, discoveries, tips and tricks!
Today I'm looking in to how the MSVC Development builds work. Yesterday I downloaded a version labeled 20220527 and from what I can tell, after looking at the source (another 3gb of disk space gone for now) https://github.com/qt/qtbase/commit/5d8f815e101da3ae9cd6a666cc097853f52b21da is the current commit.
Folly
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A lock-free ring-buffer with contiguous reservations (2019)
My interpretation is that with release semantics for the store, the 2nd read (load) in Thread 1 is actually allowed to be reordered before the release store to the hazard pointer. But they are not very explicit about it.
> So if thread 2 removing the pointer happens first, thread 1 will see a different value on its second read and not attempt to dereference it.
Thread 1 will see thread 2's remove even with release semantics for that store -- the store has a data dependency on the first load; they cannot be reordered.
> If thread 1 writes to its hazard pointer first, the garbage collector is guaranteed to see that value and not delete the node.
Yeah, this must be it. Thread 1 fails to notice the GC happened while it was writing its HP because its second load actually happened before the HP store.
Folly's hazard pointer implementation uses a release store to update the hazard pointer (here: reset_protection()), but uses some sort of SeqCst barrier between the store and the 2nd load (with acquire semantics): https://github.com/facebook/folly/blob/main/folly/synchroniz...
To set a HP on Linux, Folly just does a relaxed load of the src pointer, release store of the HP, compiler-only barrier, and acquire load. (This prevents the compiler from reordering the 2nd load before the store, right? But to my understanding does not prevent a hypothetical CPU reordering of the 2nd load before the store, which seems potentially problematic!)
Then on the GC/reclaim side of things, after protected object pointers are stored, it does a more expensive barrier[0] before acquire-loading the HPs.
I'll admit, I am not confident I understand why this works. I mean, even on x86, loads can be reordered before earlier program-order stores. So it seems like the 2nd check on the protection side could be ineffective. (The non-Linux portable version just uses an atomic_thread_fence SeqCst on both sides, which seems more obviously correct.) And if they don't need the 2nd load on Linux, I'm unclear on why they do it.
[0]: https://github.com/facebook/folly/blob/main/folly/synchroniz...
(This uses either mprotect to force a TLB flush in process-relevant CPUs, or the newer Linux membarrier syscall if available.)
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Appending to an std:string character-by-character: how does the capacity grow?
folly provides functions to resize std::string & std::vector without initialization [0].
[0] https://github.com/facebook/folly/blob/3c8829785e3ce86cb821c...
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A Compressed Indexable Bitset
> How is that relevant?
Roaring bitmaps and similar data structures get their speed from decoding together consecutive groups of elements, so if you do sequential decoding or decode a large fraction of the list you get excellent performance.
EF instead excels at random skipping, so if you visit a small fraction of the list you generally get better performance. This is why it works so well for inverted indexes, as generally the queries are very selective (otherwise why do you need an index?) and if you have good intersection algorithms you can skip a large fraction of documents.
I didn't follow the rest of your comment, select is what EF is good at, every other data structure needs a lot more scanning once you land on the right chunk. With BMI2 you can also use the PDEP instruction to accelerate the final select on a 64-bit block: https://github.com/facebook/folly/blob/main/folly/experiment...
The EF core algorithm implemented in folly [3] may be a bit faster, and implementing partitioning on top of that is relatively easy.
It would definitely compress much better than roaring bitmaps. In terms of performance, it depends on the access patterns. If very sparse (large jumps) PEF would likely be faster, if dense (visit a large fraction of the bitmap) it'd be slower.
It is possible to squeeze a bit more compression out of PEF by introducing a chunk type for Elias-Fano of the chunk complement (for very dense chunks), but you lose the operation of skipping to a given position, which is however not needed in inverted indexes (you only need to skip past a given id, and that can be supported efficiently). That is not mentioned in the paper because at the time I thought the skip-to-position operation was a non-negotiable.
[1] https://github.com/ot/ds2i/
[2] https://github.com/pisa-engine/pisa
[3] https://github.com/facebook/folly/blob/main/folly/experiment...
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How a Single Line of Code Made a 24-Core Server Slower Than a Laptop
Can't speak for abseil and tbb, but in folly there are a few solutions for the common problem of sharing state between a writer that updates it very infrequently and concurrent readers that read it very frequently (typical use case is configs).
The most performant solutions are RCU (https://github.com/facebook/folly/blob/main/folly/synchroniz...) and hazard pointers (https://github.com/facebook/folly/blob/main/folly/synchroniz...), but they're not quite as easy to use as a shared_ptr [1].
Then there is simil-shared_ptr implemented with thread-local counters (https://github.com/facebook/folly/blob/main/folly/experiment...).
If you absolutely need a std::shared_ptr (which can be the case if you're working with pre-existing interfaces) there is CoreCachedSharedPtr (https://github.com/facebook/folly/blob/main/folly/concurrenc...), which uses an aliasing trick to transparently maintain per-core reference counts, and scales linearly, but it works only when acquiring the shared_ptr, any subsequent copies of that would still cause contention if passed around in threads.
[1] Google has a proposal to make a smart pointer based on RCU/hazptr, but I'm not a fan of it because generally RCU/hazptr guards need to be released in the same thread that acquired them, and hiding them in a freely movable object looks like a recipe for disaster to me, especially if paired with coroutines https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p05...
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Ask HN: What are some of the most elegant codebases in your favorite language?
Not sure if it's still the case but about 6 years ago Facebook's folly C++ library was something I'd point to for my junior engineers to get a sense of "good" C++ https://github.com/facebook/folly
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DynaMix 2.0.0 Released
https://github.com/facebook/folly/blob/master/folly/docs/Poly.md Folly.Poly
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Deduplicating a Slice in Go
Most modern hash map designs don't do this weird shuffle with buckets and linked lists because pointer chasing is super expensive.
https://doc.rust-lang.org/std/collections/struct.HashMap.htm...
Because it's documenting the actual API in the standard this even spells out that the result has "at least the specified capacity"
https://github.com/facebook/folly/blob/main/folly/container/...
F14 is a linear map but I couldn't immediately find actual API documentation, however it should have the same property where if you ask for an F14 with specific capacity or you reserve enough capacity, that's an "at least" promise not an approximate one.
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rust-like traits on plain C++ with short macro (type erasure actually)
Or dyno or Poly or Not-Actually-Boost.TE or ...
What are some alternatives?
abseil-cpp - Abseil Common Libraries (C++)
Boost - Super-project for modularized Boost
JUCE - JUCE is an open-source cross-platform C++ application framework for desktop and mobile applications, including VST, VST3, AU, AUv3, LV2 and AAX audio plug-ins.
OpenFrameworks - openFrameworks is a community-developed cross platform toolkit for creative coding in C++.
Cinder - Cinder is a community-developed, free and open source library for professional-quality creative coding in C++.
Seastar - High performance server-side application framework
parallel-hashmap - A family of header-only, very fast and memory-friendly hashmap and btree containers.
EASTL - Obsolete repo, please go to: https://github.com/electronicarts/EASTL
Vcpkg - C++ Library Manager for Windows, Linux, and MacOS